1. Field of the Invention
The present invention relates to a thermosensitive stencil paper and a method of producing the same, and particularly to a thermosensitive stencil paper comprising a thermoplastic resin film and a porous resin layer provided thereon, from which a printing master with excellent quality can be produced by use of a heating element such as a thermal head, and also to a method of producing such a thermosensitive stencil paper.
2. Discussion of Background
There is conventionally known a thermosensitive stencil paper which is prepared by attaching a thermoplastic resin film to a substrate with an ink-permeability property, such as a porous tissue paper, with an adhesive. Further, a sticking preventing layer is overlaid on the thermoplastic resin film to prevent the thermoplastic resin film from sticking to a thermal head when perforations are made in the resin film by the application of heat thereto. In practice, a porous tissue paper is made out of hemp fiber, synthetic fiber or wood fiber, which may be used in combination, and the thus prepared tissue paper and the thermoplastic resin film are laminated with an adhesive, and then a sticking preventing layer is provided on the surface of the resin film. Such a thermosensitive stencil paper is widely used in practice.
However, the above-mentioned conventional thermosensitive stencil pacer has the following drawbacks:
(1) Since the fibers for use in the tissue paper such as a Japanese paper are partially superimposed therein, the adhesive is unfavorably accumulated at the portion where the thermoplastic resin film is brought into contact with the superimposed fibers in the tissue paper. A perforation cannot be easily formed in such a portion as mentioned above even by the application of thermal energy using the thermal head. Accordingly, the ink cannot permeate through the stencil paper at that portion, and therefore, non-printed spots tend to appear in a solid image when a solid image is printed on an image-receiving medium using a printing master prepared from the above-mentioned thermosensitive stencil paper.
(2) The fibers themselves contained in the porous tissue paper hinder the printing ink from penetrating through the stencil paper, so that the printed images tend to become uneven.
(3) Because of expensive fibers for use in the tissue paper, the cost of the thermosensitive stencil paper increases.
To solve the above-mentioned problems, there are proposed improved thermosensitive stencil papers. For example, in Japanese Laid-Open Patent Application 3-193445, a porous substrate comprising microfibers with a fineness of 1 denier or less is employed for a thermosensitive stencil paper. Although the above-mentioned problem (2) can be solved by this thermosensitive stencil paper, the problems (1) and (3) remain unsolved.
In Japanese Laid-Open Patent Application 62-198459, there is disclosed a method of producing a printing master by forming a heat resistant resin pattern which is substantially continuous on the thermoplastic resin film by gravure printing, offset printing or flexography. According to the above-mentioned printing technology, it is still difficult to print a pattern with a line width of 50 μm or less. Even though the formation of such a pattern can be achieved, the productivity is very poor and the cost is remarkably increased. In addition, when the line width is 30 μm or more, the heat resistant resin hinders the perforation from being clearly made, so that the printed image becomes uneven.
Furthermore, a thermosensitive stencil paper disclosed in Japanese Laid-Open Patent Application 3-240596 is prepared by coating a mixture of a water-dispersed polymer and finely-divided particles of colloidal silica on the surface of a thermoplastic resin film and drying the above-mentioned mixture. Thus, a porous layer is provided on the thermoplastic resin film. From the thus prepared thermosensitive stencil paper, a printing master is produced, for example, using a commercially available printing master making apparatus “PRINT GOCCO” (Trademark), made by Riso Kagaku Corporation, and then, a printed image can be obtained from such a printing master by applying a commercially available jet printing ink (Trademark “HG-4800 Ink”, available from EPSON HANBAI Co., Ltd.) to the printing master.
The pores formed in the above-mentioned porous resin layer are remarkably fine, and the density of the porous layer is very high, so that the penetrability of the printing ink through the stencil paper becomes poor. Therefore, when a thermosensitive stencil ink for general use of which viscosity is about 150 poise or more at 20° C., that is, still higher than the viscosity of the jet printing ink (with a viscosity of 0.1 poise or less at 20° C.) is employed for producing a printed image, sufficient image density cannot be obtained in practical use.
Japanese Laid-Open Patent Application 54-33117 discloses a thermosensitive stencil paper consisting of a thermoplastic resin film. By this application, the previously mentioned problems (1) to (3) caused by the conventional thermosensitive stencil papers can be solved because a porous substrate such as a tissue paper is not employed.
However, there occurs another problem that the stiffness of the stencil paper becomes low when the thickness of the thermoplastic resin film is 10 μm or less. In such a case, transportation of the stencil paper causes annoyance in the printing machine. To solve such a problem of troublesome transportation, as disclosed in Japanese Patent Publication 5-70595, it is proposed to subject a printing master prepared from the thermosensitive stencil paper to the printing operation in such a fashion that the printing master is not cut, but used in a continuous length. By winding the printing master in a continuous length round the periphery of the plate cylinder of the printing machine, it is possible to prevent the printing master from jamming in the printing machine. However, since the printing master and the feeding and taking-off unit rollers are caused to rotate along with the rotation of the plate cylinder in the printing operation, the turning moment is increased and the displacement from the rotating shaft is increased. Therefore, the printing machine is required to be heavy in this case.
On the other hand, when the thickness of the above-mentioned thermosensitive stencil paper consisting of the thermoplastic film is 5 μm or more, the thermal sensitivity becomes so poor that perforations cannot be easily formed in the film when thermal energy is applied to the stencil paper by use of a thermal head.
The inventors of the present invention have already proposed a thermosensitive stencil paper which comprises a thermoplastic film and a porous resin layer formed on one side of the thermoplastic film, as disclosed in Japanese Laid-Open Patent Application 10-24667. Printed images obtained from the above-mentioned thermosensitive stencil paper become clearer than those obtained from a printing master prepared from the conventional thermosensitive stencil paper comprising a porous tissue paper as the substrate. However, the following problems are generated:
(1) A porous resin layer (e.g. butyral resin layer) is formed by separating the resin component from a mixed solvent comprising a good solvent and a bad solvent with respect to the employed resin. In order to obtain the optimal area of pores in the obtained resin layer, it is necessary to increase the amount of bad solvent. In this case, however, the coating liquid for the formation of the porous resin layer becomes unstable, so that gelation or separation tends to easily take place in the coating liquid. Therefore, it is required to severely control the temperature of the coating liquid and the amount ratio of the good solvent to the bad solvent.
(2) It takes a considerable time to evaporate the above-mentioned good solvent so as to deposit the resin layer on the thermoplastic film. Therefore, the coating operation must be carried out very slowly to obtain an appropriate porous resin layer, thereby decreasing the production efficiency.